Ceramic femoral resurfacing head prosthesis

ABSTRACT

A ceramic femoral resurfacing head prosthesis (110) comprises a ceramic convex outer contact surface (112) engagable with an acetabulum of a patient or an acetabular cup prosthesis and a concave inner fixation surface (114) having an inner-land portion (128), the ceramic convex outer contact surface (112) and the concave inner fixation surface (114) extending to intersect each other at a rim (116). A ceramic stem (120) projects from the concave inner fixation surface (114), and is adapted to be received by a stem bore. The concave inner fixation surface (114) includes a skirt (134) which is cylindrical or substantially cylindrical, or frusto-conical or substantially frusto-conical, and at least one circumferentially elongate recess (136) at the skirt (134).

The present invention relates to a ceramic femoral resurfacing headprosthesis for use in hip resurfacing procedures. The invention furtherrelates to a method of increasing a fixation-land area of a ceramicfemoral resurfacing head and a femoral resurfacing head prosthesissystem for selective engagement of a femoral resurfacing head prosthesiswith a resected femur based on one or more characteristics of theresected femur.

Femoral resurfacing has been developed as an alternative to conventionaltotal hip replacement in a procedure for the treatment of arthritis ofthe hip, a condition which causes considerable pain and loss ofmovement. The hip is a ball and socket joint which allows the upper legto move from side to side, back to front, and to rotate. The joint ismade up of the head of the femur, the ball, which fits into theacetabulum, the socket. In a healthy hip, both the head of the femur andacetabulum are covered with cartilage which provides a smooth surfaceallowing the joint to move freely.

In general, femoral resurfacing involves the process of capping the headof the femur with a femoral resurfacing head prosthesis, attaching theprosthesis via bone cement, and fitting an acetabular cup to theacetabulum, generally using cementless fixation via a Titanium and/orHydroxyapatite plasma coating. The femoral resurfacing head prosthesisand acetabular cup are conventionally formed from metal. A femoralresurfacing head prosthesis 10 in accordance with the state of the artis shown in FIGS. 1 to 5 and typically comprises a substantiallyspherical convex outer contact surface 12, a concave inner fixationsurface 14, a rim 16 between the two surfaces defining an opening 18 anda stem 20 projecting from the concave inner fixation surface 14 andthrough the opening 18.

It has been found that a metal-on-metal resurfacing can result in theproduction of metal ions and subsequent diffusion or transport of themetal ions to the bloodstream or locality surrounding the hipreplacement prosthesis. The presence of metal ions can result inallergic reaction or other adverse health effects for the patient.

Additionally, there is a risk of partial or total detachment of thefemoral resurfacing head prosthesis from the underlying bone cement, ifthe adhesion or bonding between them is insufficient. Therefore, anincrease in the area of overlap between an inner surface of a femoralresurfacing head prosthesis to the femur is desired, in order toincrease the adhesion and thus minimise said risk. Further, bymaximising the bone volume within the head prosthesis, bone resorptionthat can occur from stress shielding may be minimised.

The present invention seeks to provide a solution to these problems.

According to a first aspect of the present invention, there is provideda ceramic femoral resurfacing head prosthesis comprising: a ceramicconvex outer contact surface engagable with an acetabulum of a patientor an acetabular cup prosthesis; a concave inner fixation surface havingan inner-land portion, the ceramic convex outer contact surface and theconcave inner fixation surface extending to intersect each other at arim; and a ceramic stem projecting from the concave inner fixationsurface, the stem adapted to be received by a stem bore, wherein a freedistal end of the stem is at, or spaced inwardly of, a plane defined bythe said rim, wherein the concave inner fixation surface includes askirt between the inner-land portion and the rim of the head prosthesis,the skirt being cylindrical or substantially cylindrical, orfrusto-conical or substantially frusto-conical, and at least onecircumferentially elongate recess at the skirt to prevent or inhibitpull-off removal of the head prosthesis.

The use of ceramic is advantageous as the production of potentiallyhazardous metal ions is prevented or limited, given the reduction in theamount of metal used for the prosthesis. Additionally, ceramics aretypically harder than most metals and therefore the wear of theprosthesis can be reduced compared to a typical arrangement, resultingin an increased longevity of the prosthesis. Hip resurfacing prosthesestypically are not formed from ceramic as ceramics are generally brittleby nature and fracture can be unpredictable.

A free distal end of the stem being at, or spaced inwardly of, the rimresults in a shorter stem than in a typical or conventional metalresurfacing arrangement. A shorter stem results in a reduced momentacting about the base of the stem and/or at any given point along thestem. This therefore reduces the amount of stress acting at or adjacentto the base and therefore improves the stress characteristics at oradjacent the base. The improvement and/or relative lack of degradationof the stress characteristics at the base of the stem is key for thepresent invention given its formation from ceramic, rather than metal asis conventional, given that, in general, ceramic has a lower ductilitythan metal. The risk of mechanical failure of the stem, and thus of theentire prosthesis, is thus reduced. Additionally, as the stem does notproject beyond the rim, the prosthesis adopts a shape whichsubstantially saves material. The device is therefore more convenientlyand cheaply produced through green machining and sintering, a methodtypically used for the production of ceramic components. Lastly, theshorter stem results in a shorter stem bore being required to be drilledfor implantation and therefore improves bone conservation.

The concave inner fixation surface may have a plurality of anti-rotationelements spaced around a circumference thereof. The ceramic stem mayinclude at least a two-part angular transition in a longitudinaldirection of the stem to meet the inner fixation surface such that theinner-land portion of the inner fixation surface at or adjacent to thestem is increased. The at least two-part angular transition may define acurve in a longitudinal direction of the stem having a non-uniformradius.

The at least two-part angular transition may include a first curved parthaving a radius in a range of 5 mm to 150 mm, a second curved parthaving a radius in a range of 10 mm to 100 mm, and a third curved parthaving a radius in a range of 0.5 mm to 2 mm. More preferably, the firstcurved part has a radius of or substantially of 50 mm, the second curvedpart has a radius of or substantially of 17.5 mm, and the third curvedpart has a radius of or substantially of 1.5 mm.

The first and second curved parts and the second and third curved partsmay be contiguous with each other. The at least two-part angulartransition may include at least one flat in a longitudinal direction ofthe stem. The at least two-part angular transition may be or may includea catenary curve. The circumferentially elongate recess may be anendless channel.

A rim of the ceramic femoral resurfacing head prosthesis defines anasymmetrical profile between the inner fixation surface and the outercontact surface with two or more different arcs having a radius in arange of 0.2 mm to 1.5 mm.

According to a second aspect of the present invention, there is provideda femoral resurfacing head prosthesis comprising: a ceramic convex outercontact surface engagable with an acetabulum of a patient or anacetabular cup prosthesis; a concave inner fixation surface, the ceramicconvex outer contact surface and the concave inner fixation surfaceextending to intersect each other at a rim; and a ceramic stemprojecting from the concave inner fixation surface and adapted to bereceived by a stem bore, wherein a free distal end of the stem is at, orspaced inwardly of, a plane defined by the said rim, wherein the concaveinner fixation surface has a plurality of anti-rotation elements spacedaround a circumference thereof. Preferably, one or more of the pluralityof anti-rotation elements have the form of a discontinuous indentation,discontinuous recess and/or discontinuous groove.

According to a third aspect of the present invention, there is provideda ceramic femoral resurfacing head prosthesis comprising: a ceramicconvex outer contact surface engagable with an acetabulum of a patientor an acetabular cup prosthesis; a concave inner fixation surface havingan inner-land portion, the ceramic convex outer contact surface and theconcave inner fixation surface extending to intersect each other at arim; and a ceramic stem projecting from the concave inner fixationsurface and adapted to be received by a stem bore, wherein a free distalend of the stem is at, or spaced inwardly of, a plane defined by thesaid rim, wherein the concave inner fixation surface includes a skirtbetween the inner-land portion and the rim of the head prosthesis, theskirt being cylindrical or substantially cylindrical, or frusto-conicalor substantially frusto-conical, at least one circumferentially elongaterecess at the skirt to prevent or inhibit pull-off removal of the headprosthesis, and a plurality of anti-rotation elements spaced around acircumference thereof.

According to a fourth aspect of the present invention, there is provideda ceramic femoral resurfacing head prosthesis comprising: a ceramicconvex outer contact surface engagable with an acetabulum of a patientor an acetabular cup prosthesis; a concave inner fixation surface, theceramic convex outer contact surface and the concave inner fixationsurface extending to intersect each other at a rim; and a ceramic stemprojecting from the concave inner fixation surface and adapted to bereceived by a stem bore, wherein a free distal end of the stem is at, orspaced inwardly of, a plane defined by the said rim, wherein the ceramicstem includes at least a two-part angular transition in a longitudinaldirection of the stem to meet the concave inner fixation surface suchthat an inner-land portion of the concave inner fixation surface at oradjacent to the ceramic stem is increased, the at least a two-partangular transition defining a curve in a longitudinal direction of thestem having a non-uniform radius.

According to a fifth aspect of the present invention, there is provideda ceramic femoral resurfacing head prosthesis comprising: a ceramicconvex outer contact surface engagable with an acetabulum of a patientor an acetabular cup prosthesis; a concave inner fixation surface, theceramic convex outer contact surface and the concave inner fixationsurface extending to intersect each other at a rim; and a ceramic stemprojecting from the concave inner fixation surface and adapted to bereceived by a stem bore, wherein a free distal end of the stem is at, orspaced inwardly of, a plane defined by the said rim, wherein the ceramicstem includes at least a two-part angular transition in a longitudinaldirection of the stem to meet the concave inner fixation surface suchthat an inner-land portion of the concave inner fixation surface at oradjacent to the ceramic stem is increased, the two-part angulartransition being or including a catenary curve.

According to a sixth aspect of the present invention, there is provideda ceramic femoral resurfacing head prosthesis comprising: a ceramicconvex outer contact surface engagable with an acetabulum of a patientor an acetabular cup prosthesis; a concave inner fixation surface; aceramic stem projecting from the inner fixation surface, the stemadapted to be received by a stem bore; a base portion of the stemcomprising at least a two-part angular transition in a longitudinaldirection of the stem to meet the inner fixation surface, therebyincreasing an inner-land portion of the inner fixation surface at oradjacent to the stem.

The extension of the inner-land portion of the inner fixation surfaceincreases an area of overlap between the inner fixation surface and thein use resected head of the femur, to which the ceramic femoralresurfacing head prosthesis is attached. This allows for an increase inan area of contact between in use bone cement or other bonding agent,typically applied between the prosthesis and the femur to adhere theprosthesis to the femur, and the inner fixation surface. An increase inthe area of contact can increase the adhesion, and therefore the risk offailure of the resurfacing by detachment of the prosthesis from thefemur is reduced. The longevity of the replacement may thus beincreased.

Preferably, the concave inner fixation surface has a plurality ofanti-rotation elements spaced around a circumference thereof. The atleast two-part angular transition may define a curve in a longitudinaldirection of the stem having a non-uniform radius.

The at least two-part angular transition may include a first curved parthaving a radius in a range of 5 mm to 150 mm, a second curved parthaving a radius in a range of 10 mm to 100 mm, and a third curved parthaving a radius in a range of 0.5 mm to 2 mm. The first curved part mayhave a radius of or substantially of 50 mm, the second curved part has aradius of or substantially of 17.5 mm, and the third curved part has aradius of or substantially of 1.5 mm. Preferably, the first and secondcurved parts and the second and third curved parts are contiguous witheach other.

The at least two-part angular transition may include at least one flatin a longitudinal direction of the stem. Preferably, the at leasttwo-part angular transition is or includes a catenary curve.

The concave inner fixation surface may include at least onecircumferentially elongate recess for surgical bone cementinterdigitation to prevent or inhibit pull-off removal of the headprosthesis. The circumferentially elongate recess may be an endlesschannel. The inner fixation surface may include a skirt between theinner-land portion and a rim of the head prosthesis, thecircumferentially elongate recess being at the skirt.

Preferably, the outer contact surface and the inner fixation surfaceextend to intersect each other at a or the rim, and a free distal end ofthe stem is at or spaced inwardly of a plane defined by the said rim. Aor the rim may define an asymmetrical profile between the inner fixationsurface and the outer contact surface with two or more different arcshaving a radius in a range of 0.2 mm to 1.5 mm. A or the rim may includeat least an outer radius to the convex outer contact surface equal to orgreater than 0.5 mm.

Preferably, the anti-rotation elements comprise one or more indentationson the inner fixation surface to receive a surgical bone cement. One ormore of the anti-rotation elements may have the form of a discontinuoussemi-toroidal indentation on the inner fixation surface. Theanti-rotation elements may be equi-angularly spaced apart on the innerfixation surface. Preferably, at least one anti-rotation elementincludes a multipart indentation having a plurality of anti-rotationzones. Each anti-rotation zone may be demarcated from the otheranti-rotation zones by discontinuities in depth of the indentation attheir respective boundary. Preferably, each anti-rotation element has acentral anti-rotation zone, and a plurality of non-central anti-rotationzones adjacent to the central anti-rotation zones. Preferably, at leasttwo non-central anti-rotation zones are separated from each other by thecentral anti-rotation zone. The central anti-rotation zone may comprisea majority of the volume of the indentation.

According to a seventh aspect of the present invention, there isprovided a method of increasing a fixation-land area of a ceramicfemoral resurfacing head without or without substantially detrimentallyaffecting a stress characteristic at or adjacent to an associated stem,the method comprising the step of forming a stem of a ceramic femoralresurfacing head prosthesis in accordance with the first aspect of theinvention with at least a two-part angular transition in a longitudinaldirection thereof to meet an inner fixation surface.

A two-part angular transition enables a first curved part to have asmaller or tighter radius of curvature, and a second curved part to havea greater radius of curvature, than the radius of curvature of thetransition between the stem and the inner fixation surface for a typicalor conventional femoral resurfacing head. A smaller radius of curvatureallows for a sharper or more abrupt transition between the innerfixation surface and the longitudinal extent of the stem which enablesan increase in the extent of the inner fixation surface. A second curvedpart having a greater bend radius reduces a stress concentration at thebase of the stem. As such, the stress characteristic adjacent to thestem is not detrimentally affected, and the risk of mechanical failureof the stem is not increased compared to currently known transitions ofmetal resurfacing heads, given that the ceramic generally has a lowerductility compared to typical metal compositions.

A radial extent of the at least two-part angular transition may beunbisectable. The at least two-part angular transition may be defined bytwo different curves. The at least two-part angular transition may bedefined by a flat and a curve in a longitudinal direction of the stem.

According to an eighth aspect of the present invention, there isprovided a femoral resurfacing head prosthesis system for selectiveengagement of a femoral resurfacing head prosthesis with a resectedfemur based on one or more characteristics of the resected femur, thefemoral resurfacing head prosthesis system comprising: a first group ofceramic femoral resurfacing head prostheses, each ceramic femoralresurfacing head prosthesis of the first group having a first convexouter contact surface with a predetermined geometry which is differentto the others within the first group, the first convex outer contactsurface being engageable with an acetabulum of a patient or anacetabular cup prosthesis; a first inner fixation surface adapted to beengaged with a femur of the patient; and a first stem projecting fromthe first inner fixation surface, the first stem having a first lateralextent adapted to be receivable by a stem bore, the first lateral extentbeing common to the first group; and at least a second group of ceramicfemoral resurfacing head prostheses, each ceramic femoral resurfacinghead prosthesis of the second group having a second convex outer contactsurface with a predetermined geometry which is different to the otherswithin the second group, the second convex outer contact surface beingengagable with an acetabulum of a patient or an acetabular cupprosthesis; a second inner fixation surface adapted to be engaged with afemur of the patient; and a second stem projecting from the second innerfixation surface, the second stem having a second lateral extent adaptedto be receivable by a stem bore, the second lateral extent being commonto the second group and different to the first group; one said ceramicfemoral resurfacing head prosthesis from the first or second groupsbeing selectively engagable with the resected femur based on the one ormore characteristics thereof.

Providing a first and second group of ceramic femoral resurfacing headprostheses, each prosthesis within each group having a convex outercontact surface having a different pre-determined geometry to everyother prosthesis within the same respective group, allows for thegeometry and/or dimensions of the convex outer contact surface to bemore closely matched to the geometry and/or dimensions of the acetabulumof a patient, without necessitating patient-specific construction ofeach ceramic femoral resurfacing head prosthesis. Advantageously, theprostheses of each of the first group and the second group respectivelymay have a common second lateral extent, the common second lateralextent of the first group being different to the common second lateralextent of the second group, so that a surgeon or other medicalpractitioner may select a second lateral extent suitable to thepatient's anatomy while reducing the variation of dimension of stem borethat the surgeon may be required to form.

The first convex outer contact surface of each ceramic femoralresurfacing head prosthesis of the first group may have a differentequatorial circumference to that of the other ceramic femoralresurfacing head prosthesis in the first group. The second convex outercontact surface of each ceramic femoral resurfacing head prosthesis ofthe second group may have a different equatorial circumference to thatof the other ceramic femoral resurfacing head prosthesis in the secondgroup. The ceramic femoral resurfacing head prostheses of the first andsecond groups each may have different equatorial circumferences. Alength and/or diameter of the first and second stems may be different.

Preferably, a rim of each ceramic femoral resurfacing head prosthesis ofthe first and second groups defines an asymmetrical profile between theinner fixation surface and the outer contact surface with two or moredifferent arcs having a radius in a range of 0.2 mm to 1.5 mm. The outercontact surface and the inner fixation surface may extend to intersecteach other at a or the rim, and a free distal end of the stem may be ator spaced inwardly of a plane defined by the said rim. The first and/orsecond inner fixation surfaces may include at least onecircumferentially elongate recess to prevent or inhibit pull-off removalof the head prosthesis.

According to a ninth aspect of the present invention, there is provideda ceramic femoral resurfacing head prosthesis comprising: a ceramicconvex outer contact surface engagable with an acetabulum of a patientor an acetabular cup prosthesis; a concave inner fixation surface, theouter contact surface and the inner fixation surface extending tointersect each other at a rim; a ceramic stem projecting from the innerfixation surface and adapted to be received by a stem bore, wherein afree distal end of the stem is at, or spaced inwardly of, a planedefined by the said rim.

The concave inner fixation surface may have a plurality of anti-rotationelements spaced around a circumference thereof. The ceramic stem mayinclude at least a two-part angular transition in a longitudinaldirection of the stem to meet the inner fixation surface such that aninner-land portion of the inner fixation surface at or adjacent to thestem is increased.

The at least two-part angular transition may define a curve in alongitudinal direction of the stem having a non-uniform radius. The atleast two-part angular transition may include a first curved part havinga radius in a range of 5 mm to 150 mm, a second curved part having aradius in a range of 10 mm to 100 mm, and a third curved part having aradius in a range of 0.5 mm to 2 mm. The first curved part may have aradius of or substantially of 50 mm, the second curved part has a radiusof or substantially of 17.5 mm, and the third curved part has a radiusof or substantially of 1.5 mm. The first and second curved parts and thesecond and third curved parts may be contiguous with each other. The atleast two-part angular transition may include at least one flat in alongitudinal direction of the stem. The at least two-part angulartransition may or may include a catenary curve.

The concave inner fixation surface may include at least onecircumferentially elongate recess to prevent or inhibit pull-off removalof the head prosthesis. The circumferentially elongate recess may be anendless channel. The inner fixation surface may include a skirt betweenthe inner-land portion and a rim of the head prosthesis, thecircumferentially elongate recess being at the skirt. A rim of theceramic femoral resurfacing head prosthesis may define an asymmetricalprofile between the inner fixation surface and the outer contact surfacewith two or more different arcs having a radius in a range of 0.2 mm to1.5 mm.

According to a tenth aspect of the present invention, there is provideda ceramic femoral resurfacing head prosthesis comprising: a ceramicconvex outer contact surface engagable with an acetabulum of a patientor an acetabular cup prosthesis; a concave inner fixation surface, theouter contact surface and the inner fixation surface extending tointersect each other at a rim; a ceramic stem projecting from the innerfixation surface and adapted to be received by a stem bore, wherein therim defines an asymmetrical profile between the inner and outer contactsurfaces with two or more different arcs having a radius in a range of0.2 mm to 1.5 mm.

The rim having an asymmetrical profile with two or more different arcsallows for the width of the rim to be increased, by reducing alongitudinal extent of the ceramic femoral resurfacing head prosthesis,whilst simultaneously maximising the inner fixation surface and reducingthe risk of damage to an external edge portion of the rim onimplantation and/or in use. The width of the rim is increased ascompared to a conventional femoral resurfacing head prosthesis byreducing the longitudinal extent of the ceramic femoral resurfacing headprosthesis while maintaining the dimensions of the outer contactsurface.

An asymmetric profile enables a radius of curvature of an external edgeof the rim to be greater than the radius of curvature of the externaledge of the rim of a conventional prosthesis. Furthermore, a radius ofcurvature of an internal edge of the rim, due to the asymmetry, may bethe same or similar as the radius of curvature of a conventionalprosthesis, for example. The similar internal edge curvature maximisesthe inner fixation surface by maintaining the abruptness or sharpness ofthe transition between the inner fixation surface and the rim, and thegradual, greater radius of curvature of the external edge reduces therisk of damage to the rim by reducing stress concentration adjacent tothe external edge rim.

The concave inner fixation surface may have a plurality of anti-rotationelements spaced around a circumference thereof. The rim may include atleast an outer radius to the convex outer contact surface equal to orgreater than 0.5 mm. A free distal end of the stem may be at, or spacedinwardly of, a plane defined by the said rim.

Preferably, the ceramic stem includes an at least two-part angulartransition in a longitudinal direction of the stem to meet the innerfixation surface such that an inner-land portion of the inner fixationsurface at or adjacent to the stem is increased. The at least two-partangular transition may define a curve in a longitudinal direction of thestem having a non-uniform radius. The at least two-part angulartransition may include a first curved part having a radius in a range of5 mm to 150 mm, a second curved part having a radius in a range of 10 mmto 100 mm, and a third curved part having a radius in a range of 0.5 mmto 2 mm. The first curved part may have a radius of or substantially of50 mm, the second curved part has a radius of or substantially of 17.5mm, and the third curved part has a radius of or substantially of 1.5mm. The first and second curved parts and the second and third curvedparts may be contiguous with each other.

The at least two-part angular transition may include at least one flatin a longitudinal direction of the stem. The at least two-part angulartransition may be or include a catenary curve. The concave innerfixation surface may include at least one circumferentially elongaterecess to prevent or inhibit pull-off removal of the head prosthesis.The circumferentially elongate recess may be an endless channel. Theinner fixation surface may include a skirt between the inner-landportion and a rim of the head prosthesis, the circumferentially elongaterecess being at the skirt.

According to an eleventh aspect of the present invention, there isprovided a ceramic femoral resurfacing head prosthesis comprising: aceramic convex outer contact surface engagable with an acetabulum of apatient or an acetabular cup prosthesis; a concave inner fixationsurface, the outer contact surface and the inner fixation surfaceextending to intersect each other at a rim; a ceramic stem projectingfrom the inner fixation surface and adapted to be received by a stembore; and at least one circumferentially elongate recess at the concaveinner fixation surface to promote surgical bone cement interdigitationand prevent or inhibit pull-off removal of the head prosthesis.

Pull-off of the head prosthesis after the hip resurfacing surgery hadbeen completed would result in a failure of the prosthesis and thus painand/or a reduction in mobility of the patient. Further surgery would benecessitated to correct the issue. Therefore, a circumferentiallyelongate recess increases or maintains a longevity of the resurfaced hipjoint.

The concave inner fixation surface may have a plurality of anti-rotationelements spaced around a circumference thereof. The circumferentiallyelongate recess may be an endless channel. The inner fixation surfacemay include a skirt between the inner-land portion and a rim of the headprosthesis, the circumferentially elongate recess being at the skirt.

The rim may define an asymmetrical profile between the inner and outercontact surfaces with two or more different arcs having a radius in arange of 0.2 mm to 1.5 mm. The rim may include at least an outer radiusto the convex outer contact surface equal to or greater than 0.5 mm. Afree distal end of the stem may be at, or spaced inwardly of, a planedefined by the said rim.

The ceramic stem may include an at least two-part angular transition ina longitudinal direction of the stem to meet the inner fixation surfacesuch that an inner-land portion of the inner fixation surface at oradjacent to the stem is increased. The at least two-part angulartransition may define a curve in a longitudinal direction of the stemhaving a non-uniform radius. The at least two-part angular transitionmay include a first curved part having a radius in a range of 5 mm to150 mm, a second curved part having a radius in a range of 10 mm to 100mm, and a third curved part having a radius in a range of 0.5 mm to 2mm. The first curved part may more preferably have a radius of orsubstantially of 50 mm, the second curved part has a radius of orsubstantially of 17.5 mm, and the third curved part has a radius of orsubstantially of 1.5 mm. The first and second curved parts and thesecond and third curved parts may be contiguous with each other. The atleast two-part angular transition may include at least one flat in alongitudinal direction of the stem.

The at least two-part angular transition may be or may include acatenary curve.

The invention will now be more particularly described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows a radial cross-section of a femoral resurfacing headprosthesis in accordance with the state of the art;

FIG. 2 shows a representation of the femoral resurfacing head prosthesisof FIG. 1 from a distal end of a stem;

FIG. 3 is a perspective representation of the radial cross-section ofFIG. 1 with stress distribution indicated, stress is here indicated bycolour with green indicating low stress and red indicating high stress;

FIGS. 4 shows an enlarged view of FIG. 1, showing the interconnection ofa stem and an inner fixation surface, stress is here indicated by colourwith green indicating low stress and red indicating high stress;

FIG. 5 shows an enlarged view of FIG. 1, showing a rim of the femoralresurfacing head prosthesis, with the value of the radius of curvatureof the internal and external edges of the rim indicated;

FIG. 6 shows a perspective representation of a first embodiment of afemoral resurfacing head prosthesis, in accordance with the first,fourth, fifth and sixth aspects of the present invention andspecifically adapted for the method in accordance with the second aspectof the present invention;

FIG. 7 shows a representation of the femoral resurfacing head prosthesisof FIG. 6 from a distal end of a stem;

FIG. 8 shows a perspective representation of a radial cross-section ofthe femoral resurfacing head prosthesis, with stress distributionindicated; stress is here indicated by colour, with green indicating lowstress and red indicating high stress;

FIG. 9 shows the radial cross-section of the femoral resurfacing headprosthesis in FIG. 8, enlarged and showing the interconnection of a stemand an inner fixation surface, with stress concentration indicated bycolour: green indicating low stress and red indicating high stress;

FIG. 10 is a similar view of the femoral resurfacing head prosthesisshown in FIG. 9, but showing in broken line a prior-art uniform-radiustransition between the stem and the inner fixation surface, as well asin overlaid solid line a multi-part transition between the stem and theinner fixation surface according to the present invention;

FIG. 11 shows an axial cross-section of the femoral resurfacing headprosthesis of the present invention overlying an axial cross-section ofthe prior-art resurfacing head, the different cross-sections beingindicated by different colours with the femoral resurfacing headprosthesis of the present invention being shown in pink, and theprior-art device being shown in grey;

FIG. 12 shows an enlargement of a portion of the rim of the femoralresurfacing head prosthesis, referenced by circle A in FIG. 11;

FIG. 13 shows the enlarged portion of the rim, shown in FIG. 12, withthe width of the rim of the present invention overlying a portion of therim of the prior-art head;

FIG. 14 shows the enlarged portion of the rim of the femoral resurfacinghead prosthesis shown in FIGS. 12 and 13, with a comparison of inner andouter lateral rim-edge radii;

FIGS. 15a, 15b and 15c show second, third and fourth embodiments of theceramic femoral resurfacing head prosthesis each embodiment beingadapted for a first group of the system in accordance with the thirdaspect of the present invention; and

FIGS. 16a, 16b and 16c show fifth, sixth and seventh embodiments of thepresent invention in accordance with each embodiment being adapted for asecond group of the system in accordance with the third aspect of thepresent invention.

Referring to FIGS. 6 to 14 there is shown a ceramic femoral resurfacinghead prosthesis 110 comprising a ceramic convex outer contact surface112, which is or is a substantially spherical surface and is generally asegment of a sphere slightly greater than a hemisphere, and a concaveinner fixation surface 114.

Although the outer contact surface and the inner fixation surface aredescribed as being convex and concave respectively, it is appreciatedthat one or both surfaces may in fact not be concave or convex and/ormay be at least in part planar or multifaceted, as necessity dictates.

The ceramic convex outer contact surface 112 and the concave innerfixation surface 114 are preferably contiguous and meet at a rim 116which thereby forms a plane and a circular or substantially circularopening 118. In this way, the rim may be planar or substantially planar.

A ceramic stem 120 projects from the concave inner fixation surface 114,preferably opposite or substantially opposite from the opening 118. Theceramic stem 120 may have a base portion 122 proximal to and at oradjacent to the concave inner fixation surface 114, a cylindrical or asubstantially cylindrical shaped body portion 124 and a frustoconical orsubstantially frustoconical shaped distal free end 126, or tip portion,distal to the concave inner fixation surface 114 and the base portion122, the body portion 124 separating the base portion 122 form thedistal free end 126.

A longitudinal extent of the ceramic stem 120 is such that the distalfree end 126 is at or spaced inwardly of the plane defined by the rim116. The ceramic stem 120 is therefore shorter than the stem 20 of theprior art femoral resurfacing head prosthesis; a direct comparison ofthe two stems can be seen in FIGS. 15a to 15c and 16a to 16c . Whilstthe extent of the ceramic stem is so described, it is appreciated thatthe longitudinal extent of the stem may be such that the distal free endof the stem extends beyond the plane defined by the rim.

The base portion 122 of the ceramic stem 120 merges, substantiallymerges or transitions into the concave inner fixation surface 114 andthe concave inner fixation surface 114 preferably has an inner-land 128which here may be at or adjacent to base portion 122 such that it may beconsidered to extend around a circumferential extent of, surround,encircle and/or be adjacent to the base portion 122. The inner-land 128is preferably planar or substantially planar and may be transverse,lateral or perpendicular or substantially perpendicular to alongitudinal extent of the ceramic stem 120. Whilst described asencircling the base portion, it is appreciated that the inner-land mayonly extend around part of the circumferential extent the base portion.

Adjacent to the inner-land 128, an intermediary portion 130 of theconcave inner fixation surface 114 may extend at an angle to the planeof the inner-land 128, and preferably at an obtuse angle. Thisarrangement is such that the intermediary portion 130 and the inner-land128 together substantially form the shape of an external surface of afrustocone with a base removed.

A plurality of, and preferably three as shown, anti-rotation elements132 may be equiangularly arranged around a circumferential extent of theconcave inner fixation surface 114, and more specifically theintermediary portion 130. Each anti-rotation element 132 may here beformed as an indentation, recess or groove in the concave inner fixationsurface 114 and a longitudinal extent of the indentation, recess orgroove may be aligned with a circumferential extent of the intermediaryportion 130. The anti-rotation elements 132 are preferably spaced apart,i.e. discontinuous from one another, to prevent rotation around acircumferential extent of the concave inner fixation surface 114. Here,preferably each anti-rotation element 132 may be elongate and may haveat least one rounded edge. Each of the anti-rotation elements 132 mayhave the form of a discontinuous semi-toroidal indentation. Thistoroidal indentation is such that a radial profile of the anti-rotationelement 132 may be a circular or semi-circular.

Additionally, whilst described as being circumferentially alignedelongate indentations, recesses or grooves, it is appreciated thatanti-rotation elements may take other forms, for example eachanti-rotation element may be a circular recess or may be a projection.Furthermore, at least one of the anti-rotation elements may include amultipart indentation having a plurality of anti-rotation zones and eachanti-rotation zone may be demarcated from the other anti-rotation zonesby discontinuities in depth of the indentation at their respectiveboundary. Each anti-rotation element may have a central anti-rotationzone and a plurality of non-central anti-rotation zones adjacent to thecentral anti-rotation zone. Although described and shown as beingpositioned in the intermediary portion, it is appreciated that theanti-rotation elements may in fact be positioned elsewhere on concaveinner fixation surface, for example on the inner-land area 128. It isalso appreciated that the ceramic femoral resurfacing head prosthesismay not necessarily include a or any anti-rotation elements.

The concave inner fixation surface 114 may further comprise a skirt 134positioned between the rim 116 and the inner-land 128, and morespecifically between the rim 116 and the intermediary portion 130. Theskirt 134 is preferably annular in shape and an axial extent of theannular shaped skirt 134 may extend parallel or substantially parallelto a longitudinal extent of the ceramic stem 120. Whilst described asextending parallel to the ceramic stem 120, it is appreciated that theaxial extent of the skirt 134 may in fact extend at an angle and towardsthe stem and as such the skirt 134 may be considered to taper or narrowin diameter towards the rim 116. Alternatively, the axial extent of theskirt 134 may in fact extend at an angle and away from the stem suchthat the skirt 134 may be considered to widen in diameter towards therim 116. The skirt 134 is preferably cylindrical or substantiallycylindrical, or frusto-conical or substantially frusto-conical.

The skirt 134 may preferably further comprise at least onecircumferentially elongate recess 136 or groove in the concave innerfixation surface 114, preferably extending around a circumferentialextent of the concave inner fixation surface 114 and/or skirt 134. Theelongate recess 136 may be an endless channel, although it isappreciated that the recess may not be endless and may only extendaround a portion of the extent of concave inner fixation surface.Additionally, there may be a plurality of circumferentially aligned andsubstantially coplanar elongate recesses 136 positioned in the skirt 134which individually extend around a portion of the circumference of theskirt 134.

The merging and/or transition of the base portion 122 of the ceramicstem 120 with or to the inner-land 128 of the concave inner fixationsurface 114 is preferably at least a two-part angular transition 138.Each part of the at least two-part angular transition 138 preferably hasa different radius or radius of curvature such that a radial extent ofthe at least two-part angular transition is unbisectable. The radius ofcurvature of each curved part is therefore different. The curved partsare not symmetrical about a plane which separates adjacent said curvedparts. One or more of the curved parts may also not be symmetrical abouta line which bisects the said curved part. The unbisectability of theradial extent of the at least two-part angular transition 138 is shownby sectioning line B1 in FIG. 9, which does not bisect the two-partangular transition 138. By contrast the bisectability of the radialextent of the single angular transition 38 of the prior art is shown bybisection line B2 in FIG. 4.

Therefore, the at least two-part angular transition 138 preferablydefines a curve in an axial or longitudinal direction or extent of thestem 120 having a non-uniform radius. Most preferably, as shown in FIG.9, the at least two-part angular transition 138 is a three-part angulartransition 140. The two- or three- part transition 138, 140 preferablyextends uniformly around the circumferential extent of the base portion122 of the ceramic stem 120. This is such that the base portion 122 ofthe stem 120 may be considered to be hyperbolic frustocone or afrustocone having a nonuniform pitch, with the wider base of thefrustocone being adjacent to the inner-land 128. A direct comparisonbetween the transition of the present invention and the transition 38 ofthe prior art can be seen in FIG. 10. The transition in accordance withthe state of the art is also shown in FIGS. 3 and 4.

The three-part angular transition 140 preferably firstly comprises afirst curved part 142. The first curved part 142 is an axial extent ofthe base portion 122 of the stem 120 being at or adjacent to thecylindrical body portion 124 having a constant, uniform or regularradius of curvature, referenced at R1 in FIG. 10. The curvature of R1 ispreferably such that it curves away from an axial direction of the stem120. The radius of curvature R1 is preferably in the range of 5 mm to150 mm and more preferably may be at, around or substantially 50 mm.Although being described as having a constant, uniform or regular radiusof curvature, it is appreciated that the first curved part may be flat,and thus have an undefinable radius of curvature, in an axial orlongitudinal extent of the stem, or may be irregularly or non-uniformlycurved. In the instance of being flat, an axial cross-section of thefirst curved part may be considered to be straight. Whilst the radius ofcurvature of the first part is given as preferably having an upperlimit, it is appreciated that the radius of curvature may in fact onlypreferably be greater than 5 mm.

The three-part angular transition 140 further comprises a second curvedpart 144. The second curved part 142 is on an axial extent of the baseportion 122 of the stem 120 being adjacent to and contiguous with thefirst curved part 142 and is distal to the cylindrical body portion 124as compared to the first curved part 142, having a radius of curvaturereferenced R2 in FIG. 10. The radius of curvature of the second curvedpart 144 may preferably be in the range of 10 mm to 100 mm, may morepreferably be in the range of 10 mm to 25 mm, may most preferably be inthe range of 15 mm to 20 mm, and may be at, around or substantially 17.5mm. Although, the second curved part 144 is described as being a curvehaving a radius of curvature of the above values, it is appreciated thatthe curve may in fact be a catenary curve thus having no singular radiusof curvature. Alternatively, the second curved part may in fact be flatin an axial or longitudinal extent of the stem, and thus have anundefinable radius of curvature in an axial or longitudinal extent ofthe stem. In this instance, an axial cross-section of the second curvedpart may be considered to be straight. Lastly the second curved part mayhave parabolic character or have any irregular or non-uniform curvature.Whilst the radius of curvature of the second part is given as preferablyhaving an upper limit, it is appreciated that the radius of curvaturemay in fact only preferably be greater than 10 mm.

The three-part angular transition 140 lastly comprises a third curvedpart 146, having a radius of curvature labelled R3. The third curvedpart 146 is on and axial extent of the base portion 122 of the stem 120being inter-positioned between and at or adjacent to both of theinner-land 128 portion and second curved part 144. The third curved part146 may preferably have a constant, uniform or regular radius ofcurvature R3. The radius of curvature R3 of the third curved part 146may preferably in the range of 0.5 mm to 2 mm, and more preferably maybe at, around or substantially 1.5 mm. Although being described ashaving a constant, uniform or regular radius of curvature, it isappreciated that the third curved part may be flat, and thus have anundefinable radius of curvature in an axial or longitudinal extent ofthe stem or may be irregularly or non-uniformly curved. In the instanceof the third curved part being flat, an axial cross-section of the thirdcurved part may be considered to be straight. Whilst the radius ofcurvature of the third part is given as preferably having an upperlimit, it is appreciated that the radius of curvature may in fact onlypreferably be greater than 0.5 mm.

Whilst the at least two-part angular transition 138 is described ashaving three angular transitions, with the first and second curved parts142, 144 being contiguous with each other and the second and third parts144, 146 being contiguous with each other, it is appreciated that it mayin fact only have two angular transitions or may have more than threeangular transitions. Whilst the above values for the radius ofcurvatures of the curved part are specified, it should be noted that theadvantage provided by the at least two-part angular transition 138 isgiven by having at least two contiguous curved parts; a curved partadjacent to the stem and a curved part adjacent to the inner-land 128,the curved part adjacent to the inner-land 128 having a tighter orsmaller radius of curvature than the radius of curvature of the curvedpart adjacent to the stem. The above values of radius of curvature R1,R2, R3 may vary and/or scale depending on the dimensions of the otherfeatures of the ceramic femoral resurfacing head prosthesis 110,particularly a diameter of the ceramic outer contact surface 112.Alternatively, the above values may not vary and/or scale depending onthe dimensions of the ceramic of the other features of the ceramicfemoral resurfacing head prosthesis 110.

It is appreciated that the transition may in fact not extend uniformlyaround the circumferential extent of the base and the number of curvedparts or radius of curvature of each part of the transition may varydepending on its circumferential position. It is further noted that theceramic femoral resurfacing head prosthesis 110 may in fact have asingular angular transition, for example, the curved part 38 in thestate of the art having a radius of curvature of 2.5 mm and indicated byR4 in FIG. 10.

The three-part angular transition 140 is so arranged to maximise thesurface area of the inner-land 128, by decreasing the lateral extentover which the transition 140 extends, whilst maintaining, improving, orrestricting or limiting a degradation of in use stress characteristics,such as stress concentration, adjacent to the base 124 of the ceramicstem 120 as compared to the prior art. This is achieved by having athird curved part 146 with a smaller radius of curvature than the radiusof curvature R4 of the prior art, allowing the transition or mergingbetween the stem 120 and the inner-land 128 to occur more proximal tothe stem 120. Any negative affect to the stress characteristics by thistighter radius of curvature is mitigated by having second and/or firstcurved parts 142, 144 having a radius of curvature greater than theprior art and greater than the third curved part 146 which reducesstress concentration. This increase of inner-land 128 whilst maintenanceof suitable stress characteristics is demonstrated by a comparison ofFIGS. 4 and 9. The lateral extent of the transition 140 between the stem120 and the inner-land 128 is indicated in FIG. 10 by X1 for the presentinvention and X2 in the prior art. Given that X1 is smaller than X2, theinner-land 128 can be shown to be extended and increased in area in thepresent invention as compared to the prior art.

The rim 116 preferably has an asymmetrical profile, the asymmetricprofile may be considered to be a lateral profile of the rim 116. Theasymmetrical profile of the rim 116, and a comparison to the symmetricalrim 116 of the prior art, may be seen in FIGS. 12 to 14.

The rim 116 here has an inner, or internal, edge 148 and an outer, orexternal, edge 150, both edges 148, 150 preferably extending around acircumferential extent of the rim 116 and thus the ceramic femoralresurfacing head prosthesis 110. The outer edge 150 may be proximal tothe ceramic convex outer contact surface 112 and the inner edge 148 maybe proximal to the concave inner fixation surface 114. Both the inneredge 148, and the outer edge 150 may be considered to be curved orsubstantially curved, the inner edge 148 having a radius of curvaturelabelled R5 in FIG. 14 and similarly R6 for the outer edge 150. Theradius of curvature of the inner edge 148 is preferably smaller than theradius of curvature of the outer edge 150. The radius of curvature ofthe inner edge 148 may preferably be in a range of 0.2 mm and 1.5 mm andmay more preferably be at, around or substantially 0.5 mm. The radius ofcurvature of the outer edge 150 is preferably in the range of 0.5 mm and2 mm, and more preferably being at, around or substantially 1 mm. Whilstthe above values are specified, it should be appreciated that the radiusof curvature may vary depending on the overall size of the ceramicfemoral resurfacing head prosthesis 110 and that the importance of thispart of the present invention is that the radius of curvature of theouter edge 150 is greater than the radius of curvature of the inner edge148. A substantially flat or planar portion may interspace the inner andouter edges 148, 150.

The effect of the lateral profile of the rim 116 being asymmetrical isthat the rim 116 may be thickened, when compared to the prior art, bythe axial extent of the ceramic femoral resurfacing head prosthesis 110being reduced, but the concave inner fixation surface 114 may still bemaximised. The smaller inner radius of curvature R5 maximises theconcave inner fixation surface 114 by reducing the axial extent of thetransition between the inner surface 114 and the distal surface of therim 116. The larger outer radius of curvature R6 reduces stressconcentration and therefore reduces the risk of fracture or failure ofpart of this component, when compared to a smaller or typical radius ofcurvature. The contrasting symmetrical profile, and therefore equalradius of curvature for both the inner and outer edge 48, 58 of the rim16 in the prior art can be seen in FIG. 5 where the radius of curvatureof both edges is labelled as R7. This radius of curvature can also beseen in FIGS. 12 and 13 where the profiles of both rims 16, 116 areshown overlain. The increase in width of the rim 116 of the presentinvention over the prior art is illustrated in FIG. 13 by a comparisonof W1, a width of the rim of the present invention, with W2, a width ofthe rim of the prior art.

Whilst the profile of the rim 116 of the ceramic femoral resurfacinghead prosthesis 110 is here described as being asymmetrical, it isappreciated that it may in fact be symmetrical, for example similar toor the same as the profile of the rim 16 in the prior art.

The ceramic femoral resurfacing head prosthesis 110 may preferably bewholly made from ceramic; however, it is appreciated that ceramicfemoral resurfacing head prosthesis may only partly be made from ceramicand that selected components, such as the stem 120, may not be made fromceramic. The ceramic selected for use in the ceramic femoral resurfacinghead prosthesis 110 may preferably be inert to the human body and shouldhave a high hardness to reduce wear. Ceramics suitable for use may inparticular be zirconia toughened alumina but also alumina, zirconia,activated alumina, bioglass, silicon nitride, zirconia or any otherceramic.

Whilst the rim 116 is here described as forming a plane it isappreciated that the rim 116 may not be substantially planar and mayundulate, for example having the form of a wave or having a sinusoidalor substantially sinusoidal circumferential extent. In the event thatthe rim 116 is not planar, a plane defined by the rim 116 may be takento be the extent of the rim 116 furthest from, closest to or at a meandistance of the rim from the inner-land portion 120. Additionally oralternatively, whilst the plane defined by the rim 116 is shown as beingperpendicular to the longitudinal extent of the ceramic stem 120, it isappreciated that the rim may not perpendicular to the longitudinalextent of the ceramic stem 120. For example, the plane defined by therim may be at an angle to the ceramic stem 120, due to an asymmetricallongitudinal extent of the skirt 134.

Referring to FIGS. 15a to 15c , there are shown variations of the firstembodiment of the ceramic femoral resurfacing head prosthesis 110together forming a first group 152 of ceramic femoral resurfacing headprostheses 110. Referring to FIGS. 16a to 16c , there are shownvariations of the first embodiment of the ceramic femoral resurfacinghead prosthesis 110 together forming a second group 154 of ceramicfemoral resurfacing head prostheses 110. Although only first and secondgroups 152, 154 are described, it will be clear that more than twogroups having the characteristics outlined hereinafter can be provided,where necessity dictates.

Each prosthesis variation of the first embodiment may be at least inpart distinguished by a differing equatorial diameter of the convexouter contact surface 112 for each ceramic femoral resurfacing headprosthesis 110. The equatorial diameter of the convex outer contactsurface 112 in mm for each variation of first embodiment is given beloweach figure as a size. For example, “Size 40” refers to an equatorialdiameter of substantially 40 mm. These variants on the present inventionare the same as the preceding embodiment with the exception that thedimensions of the stem as compared to the ceramic convex outer contactsurface 112 may differ. Elements which are similar or identical to thoseof the preceding embodiment are denoted by the same reference numberwith i to vi added to denote a variation, and further detaileddescription is omitted.

Across the first group 152, a lateral extent of the ceramic stem 120 imay be constant, common or uniform or substantially constant, common oruniform along at least a majority of a longitudinal extent of the stem.Given that the lateral extent of each stem 120 i may not be constantalong an entire longitudinal extent thereof, due at least to thetapering at the distal free end portion 126 i and the at least two-partangular transition 138 i of the base portion 122 i, the said lateralextent which is constant across the group may be taken from a same orsimilar longitudinal reference point for each ceramic stem 120 i of afirst group 152.

The longitudinal reference point may, for example, be within alongitudinal extent of the body portion 124 i, the body portion of eachceramic stem preferably having a constant or substantially constantlateral extent therealong. Alternatively, the longitudinal referencepoint may be taken to be at the base of the ceramic stem 120 i orbetween the inner-land 128 i and the body portion 124 i.

The constant or substantially constant lateral extent of each ceramicstem 120 i within the first group 152 may at least be along part of thelongitudinal extent of each ceramic stem 120 i and may preferably beconstant or substantially constant along a majority of each ceramic stem120 i of a group. Whilst a lateral extent of each ceramic stem 120 i isconstant, a longitudinal extent of each stem 120 i may vary across thegroup 152. This varying longitudinal extent may be achieved by varyingonly the longitudinal extent of the body portion 124 i of each stem 120i, in this way a longitudinal and lateral extent of the base portion 122i and the distal free end portion 126 i of each stem within the group152 may be constant or common. Alternatively, the length of the ceramicstem may be varied by also or only varying the longitudinal extent ofthe base portion 122 i, including the at least two-part angulartransition 138 i, and/or the distal free end portion 126 i. Given thenon-constant lateral extent of the base portion 122 i or distal free endportion 126 i, by varying their longitudinal extent the lateral extentof the ceramic stem 120 i may vary at these portions and thus thelateral extent of each ceramic stem 120 i within a group may not beconstant along the entirety of the longitudinal extent of the stem 120i. In the event that a lateral cross-section of the ceramic stem 120 iis not circular, the major or largest lateral dimension of the lateralextent should be constant, common or uniform across a group, in the sameor similar way as described above.

The at least two-part angular transition 138 i may be the same for eachceramic stem 120 i of each ceramic femoral resurfacing head prosthesis110 i within the first group 152. Alternatively, the at least two-partangular transition 138 i may be different for each ceramic stem 120 i ofeach ceramic femoral resurfacing head prosthesis 110 i within the firstgroup 152. This difference may be due to differing radii of curvaturefor each or any of the curved parts of the two-part angular transition138 across each femoral resurfacing head prosthesis 110 i of the firstgroup 152.

Although the longitudinal extent of each ceramic stem 120 i is describedas varying across a first group 152, it is appreciated that thelongitudinal extent of each ceramic stem 120 i may also remain constant,common or uniform or substantially constant, common or uniform.

The lateral extent of the ceramic stem 120 i may be in the range of 5 mmto 7.5 mm, and more preferably may be at, around or substantially 6.6mm. The lateral extent of the first group 152 may be given by OX inFIGS. 15a to 15c . The dimensions of the remainder of the features ofeach of the femoral resurfacing head prosthesis may vary, especially theceramic convex outer contact surface 112 i, the concave inner fixationsurface 114 i and even a length of the stem. As such, each ceramicfemoral resurfacing head prosthesis 110 i may have a differentequatorial circumference to that of the other ceramic femoralresurfacing head prostheses 110 i of the first group 152. Therefore, theceramic stems of all of the ceramic femoral resurfacing head prostheses110 i of a first group 152 may be individually selectively co-operablewith a given, and the same, stem bore, also known as the femoral bore.

In the same or similar way as above, within the second group 154, thelateral extent of the ceramic stem 120 iv may be constant, common oruniform or substantially constant, common or uniform along at least partof the longitudinal extent of the ceramic stem. Within the second group154, the said constant lateral extent may be taken at the same orsimilar longitudinal reference point as above. In the same or similarway as above, the longitudinal extent of each of the ceramic stems 120iv within the group 154 may vary or, alternatively, may not vary. Herethe lateral extent of ceramic stem 120 iv may be in the range of 7 mm to9 mm, and more preferably may be at, around or substantially 8.1 mm andtherefore may be generally greater than the lateral extent of theceramic stem 120 i of the first group 152. The lateral extent of thesecond group 154 is given by OY in FIGS. 16a to 16c . The dimensions ofthe remainder of the features of each of the femoral resurfacing headprosthesis may vary, especially the convex outer contact surface. Thedimensions of said features of the second group 154, including thelongitudinal extent of the stem 120 iv, may also vary with respect tothe dimensions of the said features of the first group 152. As such,each ceramic femoral resurfacing head prosthesis 110 iv may have adifferent equatorial circumference and/or a different longitudinalextent of the stem, to that of the other ceramic femoral resurfacinghead prosthesis 110 iv of a second group 154 and of the first group 152.

The at least two-part angular transition 138 i of the ceramic stem 120 iof the first group 152 may be the same or similar to the at leasttwo-part angular transition 138 iv of the ceramic stem 120 iv of thesecond group 154. Alternatively, the at least two-part angulartransition 138 i of the ceramic stem 120 i of the first group 152 may bedifferent to the at least two-part angular transition 138 iv of theceramic stem 120 iv of the second group 154.

However, the lateral extent of the ceramic stem 120 iv of the secondgroup 154 is different to the ceramic stem 120 i of the first group 152.Therefore, the ceramic stems of all of the ceramic femoral resurfacinghead prostheses 110 iv of a second group 154 may be individuallyselectively co-operable with a given, and the same, stem or femoralbore, but may not be individually selectively co-operable with a stem orfemoral bore suitable for use with the first group 152 and vice versa.

Therefore, a range of prostheses having differing ceramic convex outercontact surfaces 112 and concave inner fixation surface 114, but allbeing suitable for insertion into a stem bore or femoral bore of thesame dimensions, and in particular the same lateral dimension, may beselectable.

Whilst, as described, the only difference between the ceramic femoralresurfacing head prostheses 110 are the relative size of the lateralextent of the stem 120 compared to the other features, it is appreciatedthat within a group, for a given lateral extent of stem, there may beother variations. For example, there may be differing numbers ofanti-rotation elements or no anti-rotation elements between the ceramicfemoral resurfacing head prostheses of a group. Additionally, there maybe more or different shaped or sized elongate recesses to promoteadhesion to the head of the femur. Therefore, in use the surgeon mayselect from a group of femoral resurfacing head prostheses having aparticular feature and having already prepared the stem bore duringsurgery.

A surgical procedure may be performed to in use apply the ceramicfemoral resurfacing head prosthesis 110 to a patient. This may take theform of making an incision in the patient, adjacent to the hip. Thesurgical approach may be most commonly made posterior to the hip,however lateral, anterior, anterior-lateral and medial approaches arealso possible. The head portion of the femur and the acetabulum are thenexposed and the head of the femur may be dislocated from the acetabulum.

A ceramic femoral resurfacing head prosthesis 110 or femoral resurfacinghead prosthesis group 152, 154 may be selected based on the size of thefemur head and the size of the acetabulum or acetabular cup into whichthe femur is to be inserted. The head of the femur is then preparedand/or resected by shaping the femur so as to substantially correspondwith the concave inner fixation surface 114 of the selected femoralresurfacing head prosthesis. This may involve removing an edge portionof the head of the femur using a chamfer-cutting tool.

A stem bore may be drilled into a longitudinal extent of the femur, thediameter and length of the bore to be drilled corresponding to thedimensions of the stem 120 of the femoral resurfacing head prosthesis110 or femoral resurfacing head prosthesis group 152, 154 selected. If agroup 152, 154 of femoral resurfacing head prostheses 110 has beenselected then a single femoral resurfacing head prosthesis 110 isselected from that group which corresponds most closely with the size ofthe acetabulum or acetabular cup into which the resurfaced femur head isto be inserted.

Bone cement is then applied to the concave inner fixation surface 114and/or the surface of the resected femur. The femoral resurfacing headprosthesis is then positioned over the resected femur and the ceramicstem 120 is inserted into the stem bore. The bone cement bonds oraffixes the resected femur to the ceramic femoral head prosthesis. Theoptional anti-rotation elements 132 may be filled with cement andtherefore prevent rotation of the femoral resurfacing head prosthesis.Similarly, the elongate recess 136 may also be filled with cement andthereby promote surgical bone cement interdigitation to prevent orinhibit pull-off removal of the head prosthesis 110. The resurfacedceramic femoral head may then be inserted or impacted into theacetabulum or acetabular cup.

The acetabulum is then typically prepared to receive an in useacetabular cup by removing cartilage from and otherwise enlarging theacetabulum with a reamer. An in use acetabular cup, selected tocorrespond to the ceramic convex outer contact surface, is then insertedor impacted into the enlarged acetabulum and may be secured by using anadhesive such as bone cement.

The area around the hip may then be cleaned to remove any excess bone orcement, and the incision may be sealed.

It is appreciated that the above procedure may be performed in a reversesequence and the above sequence of events is given for illustrativepurposes only. For example, the procedure may clearly also be performedby first preparing the acetabulum or acetabular cup and then resurfacingthe femoral head.

It is therefore possible to provide a ceramic femoral head prosthesiswhich, through a two- or more-part angular transition of the stem toincrease an inner-land for improved bone engagement and support, has atleast comparable stress characteristics at or adjacent to a base of thestem compared to presently known non-ceramic femoral head prostheses.

Furthermore, it is also possible to provide a ceramic femoral headprosthesis with a more robust rim to prevent or limit facture, chippingor breakage, thereby significantly improving longevity.

Additionally, it is possible to provide a ceramic femoral headprosthesis with an internal profile that improves engagement of theprosthesis with the femur through surgical bone cement interdigitation,thereby reducing a likelihood of the prosthesis unseating during patientuse.

These features may be present in the ceramic femoral head prosthesiseither individually or in any combination.

It is also possible to provide a system of two or more groups ofselectable ceramic femoral head prostheses having different outersurface characteristics. By each group having a common ceramic stemdimension, profile or stem diameter, a number of tools required to fitthe selected prosthesis may be reduced.

The words ‘comprises/comprising’ and the words ‘having/including’ whenused herein with reference to the present invention are used to specifythe presence of stated features, integers, steps or components, but donot preclude the presence or addition of one or more other features,integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable sub-combination.

The embodiments described above are provided by way of examples only,and various other modifications will be apparent to persons skilled inthe field without departing from the scope of the invention as definedherein.

1. A ceramic femoral resurfacing head prosthesis comprising: a ceramicconvex part-spherical articulation surface adapted to engage with anacetabulum of a patient or an acetabular cup prosthesis; a concave innerfixation surface; and a ceramic stem projecting from the inner fixationsurface, the stem adapted to be received by a stem bore; the innerfixation surface extending axially further than the ceramicpart-spherical articulation surface to improve femoral engagement whilstpreventing or limiting in use fracture, chipping or breakage.
 2. Theceramic femoral resurfacing head prosthesis as claimed in claim 1,further comprising a rim which comprises an outer edge adjoining theceramic part-spherical articulation surface, the outer edge beingdefined by an outer arc having a first radius of curvature, and an inneredge adjoining the inner fixation surface, the inner edge being definedby an inner arc having a second radius of curvature.
 3. The ceramicfemoral resurfacing head as claimed in claim 2, wherein the inner archas an angular range of any one of 90° and substantially of 90°.
 4. Theceramic femoral resurfacing head as claimed in claim 2, wherein theouter arc has an angular range of less than 90°.
 5. The ceramic femoralresurfacing head as claimed in claim 2, wherein the inner arc has agreater angular range than that of the outer arc.
 6. The ceramic femoralresurfacing head prosthesis as claimed in claim 2, wherein the firstradius of curvature is greater than the second radius of curvature. 7.The ceramic femoral resurfacing head prosthesis as claimed in claim 6,wherein the first radius of curvature is in a range of 0.5 mm and 2.0mm.
 8. The ceramic femoral resurfacing head prosthesis as claimed inclaim 7, wherein the first radius of curvature is any one of 1.0 mm andsubstantially 1.0 mm.
 9. The ceramic femoral resurfacing head prosthesisas claimed in claim 6, wherein the second radius of curvature is in arange of 0.2 mm and 1.5 mm.
 10. The ceramic femoral resurfacing headprosthesis as claimed in claim 9, wherein the second radius of curvatureis any one of 0.5 mm and substantially 0.5 mm.
 11. The ceramic femoralresurfacing head prosthesis as claimed in claim 2, wherein an axialextent of the inner edge is less than an axial extent of the outer edge.12. The ceramic femoral resurfacing head prosthesis as claimed in claim2, wherein the centre of radius of the first radius of curvature isaxially offset with respect to the centre of radius of the second radiusof curvature.
 13. The ceramic femoral resurfacing head prosthesis asclaimed in claim 2, wherein the centre of radius of the first radius ofcurvature is radially offset with respect to the centre of radius of thesecond radius of curvature.
 14. The ceramic femoral resurfacing headprosthesis as claimed in claim 2, wherein the centre of radius of thefirst radius of curvature is any one of positioned and substantiallypositioned at a radial centre between an axial end of the inner fixationsurface and an axial end of the ceramic part-spherical articulationsurface.
 15. The ceramic femoral resurfacing head prosthesis as claimedin claim 1, further comprising a rim which comprises an outer edgeadjoining the ceramic part-spherical articulation surface, and an inneredge adjoining the inner fixation surface, a plane defined by the rimbeing non-perpendicular to a longitudinal axis of the ceramic stem. 16.The ceramic femoral resurfacing head prosthesis as claimed in claim 1,further comprising a rim which comprises an outer edge adjoining theceramic part-spherical articulation surface and an inner edge adjoiningthe inner fixation surface, the rim comprising a planar portion.
 17. Theceramic femoral resurfacing head prosthesis as claimed in claim 1,further comprising a rim which comprises an outer edge adjoining theceramic part-spherical articulation surface, and an inner edge adjoiningthe inner fixation surface, the rim being non-planar.
 18. The ceramicfemoral resurfacing head prosthesis as claimed in claim 1, furthercomprising a rim which comprises an outer edge adjoining the ceramicpart-spherical articulation surface, and an inner edge adjoining theinner fixation surface, the concave inner fixation surface comprising askirt which widens in diameter towards the rim.
 19. A ceramic femoralresurfacing head prosthesis comprising: a ceramic outer contact surfacewhich is completely or substantially convex and which is adapted to beengaged with an acetabulum of a patient or an acetabular cup prosthesis;an inner fixation surface which is completely or substantially concave,the outer contact surface and the inner fixation surface extending tointersect each other at a rim; a ceramic stem projecting from the innerfixation surface and adapted to be received by a stem bore, the innerfixation surface including an annular skirt which has an axial extent,and the rim defines an asymmetrical profile between the inner and outercontact surfaces with two or more different arcs having a radius in arange of 0.2 mm to 1.5 mm, and wherein the rim has an outer said radiuswhich is greater than an inner said radius.
 20. A ceramic femoralresurfacing head prosthesis comprising: a ceramic convex outer contactsurface adapted to engage with an acetabulum of a patient or anacetabular cup prosthesis; a concave inner fixation surface, the ceramicconvex outer contact surface and the concave inner fixation surfaceextending to intersect each other at a rim; and a ceramic stemprojecting from the concave inner fixation surface and adapted to bereceived by a stem bore, wherein the ceramic stem includes at least atwo-part angular transition in a longitudinal direction of the stem tomeet the concave inner fixation surface such that an inner-land portionof the concave inner fixation surface at or adjacent to the ceramic stemis increased, the at least a two-part angular transition defining acurve in a longitudinal direction of the stem having a non-uniformradius.